The invention relates to a method for controlling the run-up of a conveyor belt and to a drive device for a conveyor belt.
Conveyor belt systems that are long in length, for example in the range of a few kilometers, have to be run-up and braked particularly gently, in order to avoid belt oscillations and excessively high belt tensions. The rotational speed of the drive motors is therefore controlled by a control device known as a soft run-up controller, in order to enable a gentle runup of the rotational speed. For example, xe2x80x9cM. B. Singh, The Role of Drive System Technology in Maximizing the Performance and Economics of Long Belt Conveyorsxe2x80x9d, bulk solids handling, vol. 14, 1994, pp. 695-702 discloses running-up the rotational speed of the drive drum and therefore the belt speed u* with a linearly rising acceleration a* in a first time interval. In order to end the acceleration phase, in a second time interval t2 the acceleration a* is reduced to zero. This reduction is likewise carried out linearly. In other words, the first time derivative r of the acceleration a* (referred to in specialist language as the jerk) is constant and positive in a first time interval t1, and likewise constant and negative in a second time interval t2. The curve of the rotational speed (or speed v* against time) is therefore formed by quadratic functions with inverse curvature which follow each other directly, that is to say in which the first time interval t1 with a rising acceleration a*, and the second time interval t2 with a falling acceleration a* follow each other directly.
The time curve used in the known method of the setpoint value of the acceleration a*, of the setpoint value of the speed v*, and of the first time derivative r of the setpoint value of the acceleration a* (jerk) is plotted in a graph in FIG. 4.
FIG. 5 is a graph of the motor torque M which results from the known method, and also the real head and tail speed, vk and vh, respectively, of a conveyor belt having a length of 5000 m, given a time duration of the first time interval and of the second time interval of 20 s, and a maximum standardized acceleration of 0.05 sxe2x88x921, which occurs during a run-up operation. In this case, the standardized acceleration is to be understood as the ratio of the actual acceleration to the final speed of the belt. In the FIG. 5, it can be seen that significant fluctuations occur both in the motor torque M and in the conveyor belt itself In the case of long conveying lengths, this can lead to disruptive operating states.
It is also known to drive the band drive with an acceleration-time curve which runs in accordance with a sinusoidal curve. In this method, the acceleration is likewise increased continuously up to a maximum value, and reduced continuously from there. Here the first time derivative of the acceleration at the maximum, i.e., the jerk, is zero. Belt and torque fluctuations occur particularly at the end of the run-up in this method.
In both methods, the problem results that at the end of the run-up operation, overswings or underswings in the motor torque occur, which also result in a greater belt tension. Furthermore, in the case of the second method, it is not possible to move to a different rotational speed during the run-up with the same curve characteristic, since the curve characteristic is defined unambiguously by the run-up time, which is equal to the sum of the time duration of the first time interval and of the time duration of the second time interval, and of the predefined final speed.
The present invention provides a method and apparatus for controlling the run-up of a conveyor belt in which the aforementioned problems are largely avoided. In the novel method for controlling the run-up of a conveyor belt, the setpoint value of the belt speed is increased with a continuously increasing acceleration in a first time interval, and is increased with a continuously decreasing acceleration in a second time interval, with the curve of the acceleration plotted against the time having a positive curvature in the second time interval. Since, as a result of the positive, that is to say upwardly concave, curvature of the acceleration, the jerk decreases continuously in the second time interval, and belt oscillations at the end of the acceleration phase can largely be eliminated by a run-up program configured in this way.
In a preferred embodiment of the method according to the present invention, the first time derivative of the acceleration is at least approximately equal to zero at the end of the second time interval. This ensures that the conveyor belt moves particularly gently and softly to its final speed. More specifically, the second time derivative of the acceleration in the first and in the second time interval is in each case at least approximately constant, and equal and opposite. This measure makes it possible to interrupt the run-up program at any time and, during the run-up, it is possible to change the predefined final rotational speed or final speed, without resulting in a different characteristic of the acceleration-time curve.
The apparatus which facilitates the novel method is a drive device which contains a motor for driving a driving drum. A control device is assigned to the motor which permits the setpoint value of the belt speed to be controlled during the run-up of the conveyor belt in such a way that said belt speed increases continuously with continuously increasing acceleration in a first time interval, and decreases with a continuously decreasing acceleration in a second time interval. This occurs in such a way that the curve of the acceleration plotted against the time has a positive curvature.